The present invention relates to a pneumatic tire reinforced by metallic cords, more particularly to an improved metallic cord in which rubber penetration is improved without being increased in the cord diameter.
In the pneumatic tires especially radial tires for trucks, buses, light trucks and the like, steel cords are widely used as reinforcing cords for the carcass, belt, bead reinforcing layers and the like.
In recent years, in order to improve rubber penetration into such a steel cord, an invention was made, which is to use a waved filament in making a steel cord.
Generally, in the process of making a steel cord, as well known in the tire cord art, in order to prevent twisted filaments from loosing, the steel filaments are twisted together in a direction while giving each filament a rotation around its axis which rotation is reverse to the twist direction and the degree of the rotation is the same as the twist. In the finished cord, accordingly, each filament is not subjected to a rotation around its axis.
In such a conventional twisting method, if a waved filament is used, it is necessary to wave steel filaments largely in order to improve rubber penetration.
When steel filaments are waved largely, it is inevitable that the diameter of the finished cord increases and the initial elongation of the finished cord under light loads increases. As a result, the thickness and weight of cord-reinforced layers are increased and further the reinforcing effect tends to decrease.
It is an object of the present invention to provide a pneumatic tire with a cord-reinforced layer such as carcass, belt, bead reinforcing layer and the like of which metallic cords are improve in the rubber penetration without increasing the cord diameter.
According to the present invention, a pneumatic tire comprises a cord-reinforced layer made of metallic cords,
each of the metallic cords being made up of six to twelve metallic filaments whose diameter (d) is in a range of from 0.15 to 0.45 mm,
the six to twelve metallic filaments including waved filaments and unwaved filaments,
each of the waved filaments being two-dimensionally waved at a wave pitch Pw and wave height (h) before twisted, the wave pitch Pw being in a range of from 5.0 to 35.0 times the diameter (d) of the filament, and the wave height (h) being in a range of from 0.2 to 4.0 times the diameter (d) of the filament,
the metallic filaments being twisted together into the cord at a twist pitch Pc of from 10 to 40 mm so that the two-dimensionally waved filaments are each subjected to a certain rotation around its axial.
Therefore, until the cord is rubberized, the two-dimensionally waved filaments become unstable, and gaps which effectively work on rubber penetration can be formed between the filaments without increasing the thickness of the finished cord.